Zeeman modulated spectral source

ABSTRACT

Zeeman splitting of spectral lines may be used as a technique for background correction in analytical atomic spectroscopy. Conventional spectral sources suffer two main deficiencies when using this method in that the plasma in conventional lamps becomes unstable and eventually extinguishes when a magnetic field is applied, an unacceptably high magnetic field strength would be required to produce useful Zeeman splitting. In order to alleviate the above, lamps have been constructed in which emission of atomic resonance lines is achieved by sputtering or the volatilisation of sample atoms by the cathodic region of a dc discharge, followed by the excitation and emission of those atoms within the discharge and in which a magnetic field may be applied over the discharge region in parallel with the plasma causing electric field, resulting in magnetic stability.

united mates Patent 1191 Stephens July 8, 1975 ZEEMAN MODULATED SPECTRALSOURCE Primary Examiner\ incent P. McGraw [75] Inventor: Roger Stephens,Hubbards, Canada Attorney Agent or Flrm Edward Rymek [73] Assignee:Canadian Patents & Development Limited, Ottawa, Canada [57] ABSTRACT[22] Filed, Oct 23 1973 Zeeman splitting of spectral lines may be usedas a technique for background correction in analytical [2]] App]. No.:408,273 atomic spectroscopy. Conventional spectral sources suffer twomain deficiencies when using this method in [52] U S Cl 356/85 3l3/16]313/163, that the plasma in conventional lamps becomes unsta- 'fg' gf''di 4, 313/216, 313/217 ble and eventually extinguishes when a magneticfield [51] Int Cl G01j 3/30. 1 HSO is applied, an unacceptably highmagnetic field [58] Field 87' 31:5/161 163 strength would be required toproduce useful Zeeman 3l3/209 6 splitting. In order to alleviate theabove, lamps have been constructed in which emission of atomic reso-[56] References Cited nance lines is achieved by sputtering or thevolatilisation of sample atoms by the cathodic region of a dc UNITEDSTATES PATENTS discharge, followed by the excitation and emission ofl,046,082 l2/l9l2 KI'BUS et 313/163 111 3 atoms the discharge and inwhich a mag- 1,9l5,934 6/1933 Holst et al. 3l3/209 netic field may beapplied over the discharge region in 53.2 parallel with the plasmacausing electric field, result- 314051303 [0/1968 Koury et al...... 313217 magnet: 3,560,790 2/l97l Vollmer et al....... 313/217 3,676,0047/1972 Prugger et a1. 356/87 9 Chums 8 Drawmg F'gures l f 3 E 2 l I! 1 lI I I 1 I l i l A i I A ZEEMAN MODULATED SPECTRAL SOURCE This inventionrelates to spectral sources and in par ticular to novel spectral lampswhich may effectively be Zeeman modulated.

Zeeman splitting of spectral lines may be used as a technique forbackground correction in analytical atomic spectroscopy. The methodassumes a comparable background absorption of perturbed and nonperturbedcomponents of the original spectral line. whereas atomic absorption onlyoccurs on the unperturbed component due to the narrow absoption profilesof atomic spectral lines. Thus the perturbed components carryinformation on the noise levels of an analytical atomic absorptionsignal which can be used to correct and reduce such noise levels,correspondingly improving analytical sensitivity.

The difficulty of applying this method at the present time lies in thedifficulty of building Zeeman modulated spectral sources. Due to theinteractions between normal plasmas and magnetic fields. conventionalhollow cathode lamps cannot be used, since the application of the fieldsimple extinguishes the plasma.

In addition, existing hollow cathode lamps would demand unacceptablelarge magnets to give sufficiently high field strengths to produceuseful Zeeman split tings.

These problems are discussed in more detail in a paper by N. loli, P.Minguzzi and F. Strumia entitled Operation of HIgh-lntensity SpectralLamps in a Strong Magnetic Field which appeared in the Journal of theOptical Society of America, Volume 60. Number 9 September 1970. Thus,Zeeman modulated sources are generally built at present using highfrequency discharges. These require high power RF. or microwavegenerators, and usually very large magnets to produce useable Zeemansplitting.

It is therefore an object of this invention to provide a novel spectrallamp.

It is a further object of this invention to provide a spectral lampwhich may readily be Zeeman modulated.

It is yet another object of this invention to provide a spectral lampwhich may be Zeeman modulated using permanent or low powerelectromagnets.

It is further object of this invention to provide a novel spectral lampwhich is d.c. discharged.

It is yet another object of this invention to provide a novel spectrallamp in which either of the two electrodes may be used as the cathode.

These and other objects are generally achieved in the novel spectrallamp by producing a plasma causing electric field having substantiallystraight lines of electric force in a predetermined discharged regionbetween two electrodes. A magnetic field may then be applied to thedischarge region in the lamp such that the axis of the magnetic field isin parallel to these lines, avoiding plasma-field interactions. Thisresults in a plasma which is stable in the presence of the magneticfield.

The construction of the electrode assembly in the novel spectral lampswill vary depending on the spectral lines to be produced. though allassemblies are governed by the above basic principle. Embodiments willbe described for cathode materials with melting points between 600 and 1,200C, for materials with low melting points, i.e. between 200 and 600C,for materials with high melting points, i.e., above 1,200C, for liquid 2materials and finally for alkali and alkaline earth materials.

In the drawings,

FIG. I is a partial cross-section of the novel spectral lamp with oneembodiment of the electrode assembly;

FIG. 2 is a cross-section of the electrode assembly taken along line A-Ain FIG. 1;

FIG. 3 is a view of one type of electrode used in the novel lamp;

FIG. 4 is a view of a second type of electrode;

FIG. 5 is a view of a third type of electrode.

FIG. 6 is a cross-section of an electrode assembly including a liquidmaterial;

FIG. 7 is a cross-section of an electrode assembly including alkali oralkaline earth materials, and

FIG. 8 is a cross-section of the electrode assembly taken along lineB--B in FIG. 7.

As shown in FIGS. 1 and 2, the spectral lamp 1 includes a conventionalglass envelope 2 with a quartz window 3 sealed in the front end and avacuum take-off and seal tube 4 located at the other end. The novelty ofthe present spectral lamp rests with the electrode assembly 5.

The electrode assembly includes two electrodes 6 which are mountedsubstantially in parallel to one another. The assembly is sealed inplace at the back end of the envelope such that the tube may beevacuated through tube 4 and filled with a rare gas such as argon orneon at a pressure usually between 5 and 50 torr. The electrodes arethus substantially perpendicular to the envelope windows so that when anappropriate voltage is applied between the electrodes, an electric fieldhaving substantially straight lines of electric force is created,causing a discharge between the electrodes which emits a radiation beamwith predetermined spectral lines through the quartz window 3. A d.c.source is preferred, however a RF. or a microwave generator may also beused as potential sources.

The outer case of the electrodes assembly 5 may be made entirely ofmaterials such as soft iron so as to transmit a magnetic field throughthe assembly as effec tively as possible. However only walls 7 need bemade of soft iron since the poles M of the magnet used in Zeemansplitting will be located adjacent these walls. For Zeeman splitting,either a permanent magnet or an electromagnet may be used.

Finally a reflective surface 8 may be mounted at the end of theelectrode assembly 5, or as shown in FIG. 1, it may form the end wall 8of the assembly. The surface will reflect radiation emitted in thisdirection towards the front window 3.

As in all spectral line sources, the spectral lines produced depends onthe materials used in the construction of the cathode. As the differentmaterials have different melting temperatures, the electrode assemblywill vary to take this into account and have been divided into fivecategories.

Category 1 includes materials having a melting point between 600C and1,200C such as silver, copper and magnesium. This embodiment includeselectrodes 6 as shown in FIG. 3.

Electrode 6 may consist of a plane strip having a thickness 1 from 0.00linches to 0.01 inches, though only the section in the lower portion 6'immediately adjacent the discharge region need be flat. This section mayalso be necked, as shown, to raise the cathode temperature. The upperportion 6" may be necessary in some instances for very high poweroperation and will act both as a cooling fin and as an electrodeconnector. However, normally 6" is not necessary and may consist of twoterminal leads. Two similarly constructed electrodes 6 are mountedwithin the electrode assembly as shown in FIGS. 1 and 2. An asbestos/-glass combination may be used for thermal and electrical insulation.

The electrodes are mounted substantially in parallel using spacers 9consisting of glass. However, for optimum operation. the electrodesthemselves should be in physical contact with only a good thermalinsulator such as asbestos to avoid overheating the glass insulators 9(causing them to crack) or the epoxy seals between the soft iron outercase and the glass envelope (causing vacuum failure Thus asbestosspacers 10 are located between the glass spacers 9 and the electrodes 6.In addition. asbestos strips 10' are located between the electrodes 6and the outer soft iron wall 7 of the electrode assembly.

In order to permit a maximum view of the cathode surface in the forwarddirection, a slight ridge such as a fold It in the asbestos material l0(FIG. 1) or a ridge in the electrode (not shown) may be inserted at thefront of each electrode. This forces the electrode faces slightly out ofparallel.

The glass and metal portions of the electrode assembly may be sealedusing an epoxy resin, or a single casing construction may be used suchas an all metal jacket.

Finally the faces of the poles M, used to provide a desired magneticfield, are made to correspond to the width wand height Ii (FIG. 3) ofthe necked portion of the electrode.

As seen in FIGS. 1 and 2, the lamp described is sym metric havingidentical electrode construction. The electrodes are thereforeinterchangeable, and, if made from different materials. will provide fordual element operation of selecting the appropriate lamp polarity.

Category 2 includes materials having a melting point between 200C and600C such as lead, cadmium and zinc. The electrode assembly is similarto that described above except for the electrode structure which isshown in FIG. 4. The electrode 6 is made from a good heat conductor suchas brass. The low melting point material 12 is deposited over an area ofthe lower portion 6' of the electrode. This area again corresponds tothe area of the faces of the magnet poles M. The lower portion 6' may,in addition, be extented downward and connected to heat sinks on theexterior of the electrode assembly. This is particularly useful if thematerial concerned has a low wavelength resonance line, requiring highexcitation energy and a correspondingly high energy cathode discharge.

The parallel electrodes may be spaced as described with regard tocategory I. and dual element operating lamps may be constructed usingthe above electrodes because of the symmetry of the lamp.

Category 3 includes materials having a melting point above 1,200C suchas iron, cobalt and nickel.

The cathode for these materials should have as high a temperature aspossible during operation and therefore as shown in FIG. 5, theelectrode 6 is not extended outside the electrode assembly. Theelectrode material 13 is electrically connected to a rigid wire 14, suchas tungsten, having a diameter of from 0.001 inch and 0.01 inch. Thewire 14 minimises heat loss and also LII provides electrical contact.The electrode material should be as thin as possible consistent withmechanical stability. However metals such as chromium or manganese,whose mechanical characteristics do not permit them to be readily formedinto thin sheets or foil, can be deposited on a base having a highmelting point. Finally the electrodes 6 are mounted in parallel withspacers as in FIG. 1. The asbestos insulator 10 may be cut away at thecentre. Once again, because of the symmetry of the two electrode lamp,dual element operating lamps may be constructed.

Category 4 includes liquid elements such as mercury. The electrodes forsuch a material are shown in FIG. 6. The lamp includes a quartz jacket15 having a pool of the liquid material 16. Two electrodes I7, 18 madeof refractory wire such as tungsten are sealed within quartz capillaries19. These capillaries provide for a discharge only in the desiredregion. These electrodes are then sealed within the quartz jacket 15 inparallel to each other and with one electrode in electrical contact withthe liquid material 16. The quartz jacket may have partial sleeves ofsoft iron, aligned such that the poles of a magnet may be applied to thelamp, maintaining the magnetic and electric fields in parallel. The lampmay also include a reflective surface on the inside or outside of thequartz jacket 15 to direct the radiation.

As an alternative, thin metal sheet in parallel may be used aselectrodes in place of the wires, to improve lamp intensity andstability. The cathode 17 which is dipped in the liquid material 16,vaporises the liquid into the discharge region between the electrodesproviding the predetermined radiation.

Category 5 includes the alkali and alkaline earth elements. Electrodesfor some of these elements may be made in the same fashion as incategory 2, FIG. 4, where suitable support materials exist, howevergenerally an electrode assembly as shown in FIGS. 7 and 8 will be used.The electrode assembly 5 includes an outer casing which may have atleast two walls 7 made of soft iron so as to transmit a magnetic fieldas effectively as possible. The cathode is formed by packing the cathodematerial 22 into the end of a bored metal rod 21, such as copper, whichprovides mechanical support. The bore having a diameter of approximately1/60 inch. The rod 21 is therefore in intimate contact with the element22 and acts as a thermal heat sink as well as an electrical contactterminal. A pyrex or ceramic tube 20 is fitted over the entire length ofthe rod 21 such to prevent discharge from any part of the rod except theend where the cathode material 22 is exposed. The cathode is mountedwithin the assembly 5. The anode consists of a refractory wire 24, suchas tungsten covered by a second pyrex or ceramic tube 23 which alsoprevents undesired discharge. The end of the wire is bent so as to havea face with a width equal to the diameter of element 22, as shown onFIG. 8. The anode is mounted within the assembly 5, such that a linedischarge of width equal to the diameter of element 22 is effectedbetween the electrodes. The electrodes are also aligned such that theline discharge is perpendicular to walls 7.

In all the lamps described, emission of atomic reasonance lines isachieved by sputtering or volatisation of sample atoms by the cathodicregion of a discharge, fol lowed by excitation and emission of thoseatoms within the discharge. Magnetic stability is achieved in all casesby arranging the axis of the applied magnet field to be in parallel withelectric field causing the plasma, over the discharge region. Thiseliminates net plasma-field interactions.

The lamps are normally run in for a period of up to about 5 hours. Thisconsists of initial high current operation under argon or neon for 2 or3 minutes, followed by evacuation and re-filling. The cycle is repeatingat gradually reduced lamp currents and increasing running times until astable output is obtained. This conditions the cathode, and the lamp isthen sealed. For the lamps which are symmetric, i.e., two identicalelectrodes, either electrode may act as cathode for dual operation andtherefore each electrode should be conditioned separately while it isacting as cathode. Finally, with very volatile elements some anodicsputtering may occur, leading to the emission ofa mixture of lines fromboth the anode and the cathode.

I claim: 1. A spectral source comprising: first and second spacedelectrodes mounted within said spectral source, said first and secondelectrodes adapted to be connected across a potential source to producea plasma causing electric field having substantially parallel straightlines of electric force in a predetermined discharge region between saidelectrodes and at least one of said electrodes including a materialadapted to emit radiation having predetermined spectral lines; and

means adapted to apply a magnetic field to said discharge region withthe axis of the magnetic field substantially parallel to said lines ofelectric force, for producing Zeeman splitting of said spectral lines.

2. A spectral source as claimed in claim 1 in which at least a portionof each of said electrodes includes a plane surface; said electrodespositioned to provide the discharge region between substantiallyparallel plane surfaces.

3. A spectral source as claimed in claim 2 in which each of saidelectrodes consists entirely of said predetermined radiation emittingmaterial.

4. A spectral source as claimed in claim 2 in which said plane surfaceof each electrode is coated with said predetermined radiation emittingmaterial.

5. A spectral source as claimed in claim 2 in which the first electrodeincludes a first predetermined radiation emitting material and thesecond electrode includes a second predetermined radiation emittingmaterial.

6. A spectral source as claimed in claim 2 in which the plane surface ofthe electrodes are positioned slightly out of parallel to provide amaximum view of the surfaces in one direction.

7. A spectral source as claimed in claim I wherein:

said predetermined radiation emitting material is liquid,

said first electrode includes a rigid wire with one end in electricalcontact with said material;

said second electrode includes a second rigid wire mounted in parallelto the first wire, to provide a discharge region between saidelectrodes.

8. A spectral source as claimed in claim 1 wherein:

said predetermined radiation emitting material is an alkali or analkaline earth element;

said first electrode includes a first non-conducting cylinder; and anelectrically conducting rod. axially bored along a portion of itslength, positioned within said first cylinder, with said material packedwithin said bore;

said second electrode includes a second nonconducting cylinder; and arigid electrically conducting wire positioned within said secondcylinder;

said electrodes mounted within the spectral source to provide a linedischarge region between the end of said first electrode and the end ofsaid second electrode.

9. A spectral source as claimed in claim 1 wherein:

said predetermined radiation emitting material is liquid:

said first electrode includes a thin rigid metal sheet with one end inelectrical contact with said material and said second electrode includesa second rigid metal sheet mounted in parallel to said first sheet, toprovide a discharge region between said electrodes.

1. A spectral source comprising: first and second spaced electrodesmounted within said spectral source, said first and second electrodesadapted to be connected across a potential source to produce a plasmacausing electric field having substantially parallel straight lines ofelectric force in a predetermined discharge region between saidelectrodes and at least one of said electrodes including a materialadapted to emit radiation having predetermined spectral lines; and meansadapted to apply a magnetic field to said discharge region with the axisof the magnetic field substantially parallel to said lines of electricforce, for producing Zeeman splitting of said spectral lines.
 2. Aspectral source as claimed in claim 1 in which at least a portion ofeach of said electrodes includes a plane surface; said electrodespositioned to provide the discharge region between substantiallyparallel plane surfaces.
 3. A spectral source as claimed in claim 2 inwhich each of said electrodes consists entirely of said predeterminedradiation emitting material.
 4. A spectral source as claimed in claim 2in which said plane surface of each electrode is coated with saidpredetermined radiation emitting material.
 5. A spectral source asclaimed in claim 2 in which the first electrode includes a firstpredetermined radiation emitting material and the second electrodeincludes a second predetermined radiation emitting material.
 6. Aspectral source as claimed in claim 2 in which the plane surface of theelectrodes are positioned slightly out of parallel to provide a maximumview of the surfaces in one direction.
 7. A spectral source as claimedin claim 1 wherein: said predetermined radiation emitting material isliquid, said first electrode includes a rigid wire with one end inelectrical contact with said material; said second electrode includes asecond rigid wire mounted in parallel to the first wire, to provide adischarge region between said electrodes.
 8. A spectral source asclaimed in claim 1 wherein: said predetermined radiation emittingmaterial is an alkali or an alkaline earth element; said first electrodeincludes a first non-conducting cylinder; and an electrically conductingrod, axially bored along a portion of its length, positioned within saidfirst cylinder, with said material packed within said bore; said secondelectrode includes a second non-conducting cylinder; and a rigidelectrically conducting wire positioned within said second cylinder;said electrodes mounted within the spectral source to provide a linedischarge region between the end of said first electrode and the end ofsaid second electrode.
 9. A spectral source as claimed in claim 1wherein: said predetermined radiation emitting material is liquid; saidfirst electrode includes a thin rigid metal sheet with one end inelectrical contact with said material and said second electrode includesa second rigid metal sheet mounted in parallel to said first sheet, toprovide a discharge region between said electrodes.